This video is dedicated to an experiment carried out in 1914 by two German physicists, James Franck y Gustav Hertz, showing the existence of discrete atomic energy levels. The original experiment is recreated by using a tube containing mercury gas and two separated electrodes (cathode and anode); the application of a voltage between the electrodes produces a direct electric current whose electrons are scattered by the mercury atoms. It turns out that for certain (discrete) values of the voltage the electric current intensity registered at the anode drops sharply. The explanation is that for any of these voltages the energy of an electron of the current is equal to (or equal to an integer multiple of) the difference between the energies of two atomic levels. Then, an electron of the current may give its energy to a mercury atom in the lower energy level producing its excitation to the upper energy level. Educational Innovation Project UV-SFPIE_GER17_588585 funded by Vicerectorat d'Ocupació i Programes Formatius, Universitat de València. Production: Servei de Formació Permanent i Innovació Educativa (http://www.uv.es/sfpie).